Plastids are a heterogeneous family of organelles found ubiquitously in plants and algal cells. Most prominent are the chloroplasts, which carry out such essential processes as photosynthesis and the biosynthesis of fatty acids as well as of amino acids. Chloroplasts are complex organelles composed of six distinct suborganellar compartments: three different membranes (the two envelope membranes and the internal thylakoid membranes) and three compartments (the innermembrane space of the envelope, the stroma and the thylakoid lumen). More than 98% of all plastid proteins are translated on cytosolic ribosomes. Such proteins are posttranslationally targeted to and imported into the organelle. For a review, see, Jarvis et al. (2008) New Phytologist 179:257-285. Such translocation is mediated by multiprotein complexes in the outer and inner envelope membranes called TOC (Translocon at the Outer envelope membrane of Chloroplasts) and TIC (Translocon at the Inner envelope membrane of Chloroplasts). See, Soll et al. (2004) Nature Reviews. Molecular Cell Biology 5:198-208, Bedard et al. (2005) Journal of Experimental Botany 56:2287-2320, Kessler et al. (2006) Traffic 7:248-257, and Smith et al. (2006) Canadian Journal of Botany 84:531-542. Once the chloroplast precursor enters the stroma, the transit peptide is cleaved off, leaving the remaining part of the protein to take on its final conformation or engage one of a number of different sorting pathways. See, Keegstra et al. (1999) Plant Cell 11:557-570, Jarvis et al. (2004) and Gutensohn et al. (2006) Journal of Plant Physiology 163:333-347.
Methods and compositions are needed to allow heterologous polypeptides to be targeted to the chloroplast.